The invention concerns a synchronous transmission system for digital signals to be combined into a multiplex signal.
The invention also concerns a monitoring device for a synchronous transmission system and a method for localizing a fault in a transmission path between a data source and a data sink in a synchronous transmission system.
A synchronous transmission system for digital signals to be combined into a multiplex signal is, for example, a transmission system for the Synchronous Digital Hierarchy (SDH) or a transmission system for synchronous optical networks according to the SONET standard that has been adopted by the American National Standards Institute. In a transmission system for the Synchronous Digital Hierarchy and in a SONET transmission system, signals to be transmitted are combined according to a predefined pattern into a multiplex signal, and structured according to frames. A frame of this kind is referred to in SDH as a Synchronous Transport Module (STM); it is described, for example, in ITU-T Recommendation G.707 (Draft) (11/95). The frame consists of 270 columns and 9 rows. Each column of a row contains one byte: the xe2x80x9csection overheadxe2x80x9d (SOH) for control and fault detection data is provided in columns 1 through 9 of lines 1 through 3 and 5 through 9. A pointer management unit, which is referred to as an xe2x80x9cAU pointer,xe2x80x9d is provided in the fourth row of columns 1 through 9. The remaining columns and rows contain the useful information (the xe2x80x9cSTM-1 payloadxe2x80x9d). An STM-1 payload is, for example, housed in a virtual container VC-4 which comprises a region for payload data and a region for control data (path overhead (POH)).
In a transmission system, individual network elements are connected to one another via one or more transmission media (e.g. copper cables, fiber optic cables). One of the network elements is assigned the function of a control center which monitors and controls the transmission system. The control center and its functions are usually regarded as a network management system (NMS). This NMS, for example, detects any interruption in the transmission path, diverts signal traffic onto an error-free transmission path, and causes the interruption to be repaired. The functions of network management systems are known, for example, from O. Gonzales Soto et al., xe2x80x9cNetzplanung und -managementxe2x80x9d [Network planning and management], Elektrisches Nachrichtenwesen (Alcatel), 4th quarter 1993, pp. 366-377. This describes the basic principles of network level management (NLM). Transmission paths and associated connections are checked by NLM processes for their availability and performance capability. NLM processes are responsible for assigning certain messages of the network elements to specific elements in the network level.
At one network element, a signal intended for a different network element is inserted into the transmission path; this network element represents a signal source. The network element which receives this signal, on the other hand, represents a signal drain. The NMS monitors whether the signal arrives at the signal drain, and whether faults have occurred in the signal. It is evident from this that the transmission system and the NMS operate most effectively if the transmission system is the responsibility of a single system operator which has equal access to the signal source and the signal drain.
The region of a virtual container for the control data (the POH) is described in detail in section 9.3, pages 62 through 68 of the aforesaid ITU-T Recommendation G.707 (Draft). The POH of, for example, a VC-3 or VC-4 comprises nine bytes, which are designated J1, B3, C2, G1, F2, H4, F3, K3, and N1. The G1 byte (see section 9.3.1.4) is used to transmit data which provide information as to the status of the transmission path. The fifth bit of the G1 byte is set (logical state 1) if the transmission path is defective, and thus offers a possibility for monitoring the transmission path. The fifth bit is referred to as the remote defect indication (RDI).
A further possibility for monitoring the transmission path is indicated in Annex C, pages 91 through 103, and Annex D, pages 104 through 107, of ITU-T Recommendation G.707. The tandem connection monitoring (TCM) protocol disclosed there uses the N1 byte of the POH, which is referred to as the tandem connection overhead (TCOH).
In the article xe2x80x9cAutomated Network Connection Tracing and Data Gathering Methods in the SDH Networkxe2x80x9d (IEEE Transactions on Communications Vol. 42, no. 2/3/4, February/March/April 1994, pp. 1065-1075) of Y. Yasuda et al., two methods are indicated for testing connections in an individual transmission system: the ID Stamp method and the Path Trace method. Both methods use the single central network management system which is allocated to the one individual transmission system in order to collect and analyze monitoring data and coordinate the monitoring functions. These methods are therefore not to be used in a transmission system which is made up of several individual transmission systems each having its own network management system.
In known transmission systems for the Synchronous Digital Hierarchy which are made up of several individual transmission systems each having its own network management system, no features are provided for localizing errors, which occur.
It is the object of the Invention to provide a synchronous transmission system and a method in which effective and economic error localization is guaranteed. A synchronous transmission system having monitoring devices, which can modify at least one predetermined byte of control data during transmission of a frame, achieves this object, and a method including the steps of creating a data channel between monitoring device and signal source, and using the channel to report any transmission fault to the signal source, also achieves this object. Another object of the invention is to provide a monitoring device for a synchronous transmission system.
One advantage of the invention is that exact error location, and identification of the type of error, are possible.